<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  用户名：<input type="text" id="username">
  <p></p>
  <script>
      const p =document.querySelector('p')

    function handle(data) {
      input.style.border = '1px solid red'
      p.innerHTML = data.msg
    }
    const input = document.querySelector('#username')
    input.onblur = function() {
      let username = this.value
      const script = document.createElement('script')
      script.src = 'http://127.0.0.1:8000/check-username'
      document.body.appendChild(script)
    }
  </script>
</body>
</html>